Two-part molded plungers are available to drive and draw fluid through syringes. These plungers are made by fitting a molded soft rubber cover over a separately molded hard plastic core. The rubber cover is typically assembled by hand over the hard plastic core to form the completed plunger. When the plunger is inserted into a syringe, the soft rubber cover creates a seal with the inner circumference of the syringe so that fluid may drawn into or driven out of the syringe. The hard plastic core provides support for the rubber cover to prevent excessive deformation and possible leakage.
Typically, that portion of the hard plastic core which supports the rubber cover against fluid pressure, must have a smooth exterior profile conforming to the profile of the rubber cover. This is necessary in order for the hard plastic core to support the rubber cover against fluid pressure generated when the plunger is driven into the syringe. Otherwise, the rubber cover would extrude into voids or depressions in the hard plastic core, weakening or damaging the rubber cover.
Several problems have plagued prior art two-part plungers. First, hand assembly of the two parts of the plunger is expensive and time-consuming. Second, because the portion of the hard plastic core which supports the rubber cover against fluid pressure must have a smooth exterior profile, in some applications there are regions of the hard plastic core including large quantities of solid plastic. These large regions of molten plastic are difficult to adequately cool and cure in a conventional injection molding process, leading to delay in manufacturing and/or quality variations as a result of some cores being removed from the mold before the plastic forming the core is adequately cured.
It would be desirable to provide a plunger that can be manufactured by a fully automated process, in which there is reduced potential for manufacturing delay and quality variation resulting from regions of uncured plastic.